I should point out that while it's simple to redefine the behavior to have values of %a overwrite %b, or vice verse, you can get more customizable behavior using *cough*my module*cough* Hash::Merge. One included way with this module would have values of equivalent keys to be placed in an anon array, but the module allows for specific settings.

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Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain
"I can see my house from here!"
It's not what you know, but knowing how to find it if you don't know that's important